1. Field of the Invention
The present invention relates to a negative-working lithographic printing plate precursor comprising a novel polysiloxane.
2. Description of the Related Art
Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
The so-called “analogue” printing plates are generally obtained by first applying a so-called computer-to-film (CtF) method, wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an imagesetter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called “computer-to-plate” (CtP) method has gained a lot of interest. This method, also called “direct-to-plate”, bypasses the creation of film because the digital document is transferred directly to a printing plate precursor by means of a platesetter. A printing plate precursor for CtP is often called a digital plate.
Digital plates can roughly be divided in three categories:
(i) silver plates, working according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates containing a photopolymerizable composition that hardens upon exposure to light and (iii) thermal plates of which the imaging mechanism is triggered by heat or by light-to-heat conversion.
Photopolymer plate precursors can be sensitized by blue, green or red light (i.e. wavelength range between 450 and 750 nm), by violet light (i.e. wavelength range between 350 and 450 nm) or by infrared light (i.e. wavelength range between 750 and 1500 nm). Lasers have become the predominant light source used to expose photopolymer printing plate precursors. Typically, an Ar laser (488 nm) or a FD-YAG laser (532 nm) can be used for exposing a visible light sensitized photopolymer plate precursor. The wide-scale availability of low cost blue or violet laser diodes, originally developed for data storage by means of DVD, has enabled the production of platesetters operating at shorter wavelength. More specifically, semiconductor lasers emitting from 350 to 450 nm have been realized using an InGaN material. For this reason, photopolymer plates having their maximal sensitivity in the 350 nm to 450 nm region have been developed during the last years. An advantage of violet photopolymer technology is the reliability of the laser source and the possibility of handling the non-developed photopolymer plate precursors in yellow safelight conditions. The use of infrared lasers also became more important in the last years, for example the Nd-YAG laser emitting around 1060 nm but especially the infrared laser diode emitting around 830 nm. For these laser sources, infrared sensitive photopolymer plate precursors have been developed. The major advantage of infrared photopolymer technology is the possibility to handle the non-developed photopolymer plate precursors in daylight conditions.
Typically, a photopolymer plate precursor contains a photopolymerizable coating including a polymerizable compound, a polymerization initiator and a binder.
The support of the lithographic printing plates are typically aluminum supports which have a hydrophilic surface or on which a hydrophilic layer has been provided. This hydrophilic surface and/or layer should improve the water acceptance of the non-printing areas of a lithographic printing plate and the repulsion of the printing ink in these areas. During developing the soluble portions of the photopolymerizable coating should be easily removed whereby the surface of the support remains residue-free so that clean background areas are obtained during printing. Without such a residue-free removal, so-called toning would occur during printing, i.e. the background areas would accept printing ink. However, at the same time the adhesion of the image-areas on the aluminum surface should be as high as possible. Therefore, in the art there have been several technological developments, trying to solve these issues. In several approaches, reactive interlayers or sublayers and/or support treatments have been designed to optimize the lithographic contrast by increasing the hydrophobicity and/or adhesion at the image-areas, while remaining or increasing the hydrophilicity at the non-image areas.
EP 256 256 discloses a method to produce a grained and anodized aluminum substrate which is surface treated with a hydrolyzed and condensed silane for printing plate applications.
U.S. Pat. No. 6,030,748 discloses a photosensitive lithographic printing plate having an intermediate layer between the photosensitive layer and the substrate including an inorganic polymer obtained by hydrolysis and polycondensation of a specific silane coupling agent in a solution containing a phenol having a molecular weight of 1000 or less or an organic phosphoric acid compound.
EP 418 575 discloses the surface treatment of a grained and anodized aluminum substrate with a mixture of a fluoride and a hydrolyzed and condensed silane.
U.S. Pat. No. 5,807,659 discloses a negative working lithographic printing plate, wherein the lithographic support is functionalized with a group having an unsatured bond, capable of undergoing a radical addition reaction, and a silicium atom which is covalently bonded to an aluminum atom or a carbon atom of the support via an oxygen atom.
WO 2009/154740 discloses a substrate provided with an interlayer including a specific trialkoxysilane polyethylene glycol acrylate.
U.S. Pat. No. 5,204,143 discloses a process for functionalizing a grained and anodized aluminum by treating its surface with a solution containing an inorganic polymer obtained by hydrolysis and polycondensation of an organometallic compound including an organic functional group.
WO 2006/021446 discloses phosphono-substituted siloxanes including a secundary or tertiary amino group as hydrophylic layer on an aluminum oxide support. WO 2006/077048 discloses phosphono-substituted siloxanes including a secundary or tertiary amino group suitable as interlayer material in lithographic substrates and for post-treating developed lithographic printing plates.
All the above discussed approaches to improve lithographic properties are focused on a separate surface treatment of a lithographic support and/or on the design of an intermediate layer on the lithographic surface. The surface treatment of a support and the application of an intermediate layer on the surface of a support which involves an additional coating step from a solvent, are both cumbersome operations. Therefore, from both a practical and an economical point of view, it would be advantageous to have printing plates based on photopolymerisation which have an improved lithographic latitude without the need for an additional treatment of the grained and anodized aluminium support and/or for an extra layer between the support and the photopolymer layer.